ACCELERATION IS DIFFERENT!

Einstein noticed something special about accelerating reference frames compared with those that are not accelerating. This difference is apparent if we consider the situation of an observer who is enclosed in a chamber that is completely sealed and opaque.
Suppose that you are in a space ship in which the windows are covered up and the radar and navigational equipment have been placed on standby. There is no way for you to examine the surrounding environment and determine where you are, how fast you are moving, or in what direction you are moving. However, you can tell whether or not the ship is accelerating. This is so because acceleration always produces a force on objects inside the ship.
When the ship’s engines are fired and the vessel gains speed in the forward direction, all the objects in the ship (including your body) perceive a force directed backward. If the ship’s retro rockets are fired so that the ship slows down (decelerates), everything in the ship perceives a force directed forward. If rockets on the side of the ship are fired so that the ship changes direction without changing its speed, this too is a form of acceleration and will cause everything inside the ship to perceive a sideways force. Some examples are illustrated in Fig. 1.
The greater the acceleration, or change in velocity, to which the space ship is subjected, the greater is the force on every object inside it. If m is the mass of an object in the ship (in kilograms) and a is the acceleration of the ship (in meters per second per second), then the force F (in newtons) is their product:
F = ma
This is one of the most well-known formulas in physics. 
This acceleration force occurs even when the ship’s windows are covered up, the radar is switched off, and the navigational equipment is placed on standby. There is no way the force can be blocked out. In this way,

Fig. 1. When a vessel in deep space is not accelerating, there is no force on the objects inside. When the ship accelerates, there is always a force on the objects inside.

Einstein reasoned, it is possible for interstellar travelers to determine whether or not their ship is accelerating. Not only this, but they can calculate the magnitude of the acceleration as well as its direction. When it comes to acceleration, there are, in a certain sense, absolute reference frames in the cosmos.